first i would say THANK YOU for this very great plugin! I really love it!

Now I have one small problem. When I set the plugins config to
Most popular posts over the last 10 days (Wordpress.com Stats Plugin is installed)
The Plugin shows me a lot of very old posts. And I'm shure that these old posts never been watched so much times in the last 10 days. there are a lot of other posts more watched...

Hello to you in Switzerland! I've looked into your bug report and investigated the relevant, code, and I can't find anything that seems amiss. The Featurific for Wordpress plugin delegates nearly all of the work related to popular posts to the Stats plugin. There are three things to check:

1. Make sure you just have '10' (without quotes) typed into the input box. Anything extraneous characters might be throwing things off.

2. Make sure that the posts that are showing up are, in fact, not the most popular posts. Perhaps you could go into the Stats plugin and manually identify the most popular posts over the last ten days.

3. File a bug report with the authors of the Stats plugin.

Obviously, option #3 is a last resort, but if the first two options are unfruitful, that's your best bet for getting the problem resolved.